OFDMA
Orthogonal frequency-division multiplexing (OFDM) is a modulation technique used at the physical layer (PHY) of a number of wireless networks, e.g., networks designed according to the well-known IEEE 802.11a/g and IEEE 802.16/16e standards. Orthogonal frequency-division multiple access (OFDMA) is a multiple access scheme based on OFDM. In OFDMA, separate sets of orthogonal sub-channels (“tones”), and time slots are allocated to mobile stations (MSs) so that the MSs can communicate concurrently. OFDMA is widely adopted in many next generation cellular systems such as 3GPP Long Term Evolution (LTE) and IEEE 802.16m due to its effectiveness and flexibility in bandwidth allocation.
OFDMA Bandwidth Allocation
The radio frequency (RF) spectrum (bandwidth) is a scarce resource in wireless communications. Therefore, an efficient use of the bandwidth is needed. The rapid growth of wireless applications and subscriber mobile states, e.g., mobile telephones, have called for a good radio resource management (RRM) scheme that can increase the network capacity and, from a commercial point of view, save deployment cost. Consequently, developing an effective bandwidth allocation scheme for OFDMA is of significant interest for industry.
Effective bandwidth allocation has to consider the limited radio spectrum, the vast area to be covered, and large number of MSs. In other words, the same bandwidth channels must be reused in multiple geographical areas or cells. Typically, the bandwidth is allocated in a coordinated manner by base stations at the approximate centers of the cells using some infrastructure. This will inevitably incur inter-cell interference (ICI) when MSs in adjacent cells use the same spectrum. ICI is the predominant performance-limiting factor for wireless cellular networks.
Fractional Frequency Reuse
Fractional frequency reuse (FFR) is one techniques to achieve a balance between spectral efficiency and interference reduction. FFR partitions a cell into a center zone near the base station and an edge zone at the periphery of the cell. FFR allocates frequency sub-channels to MSs in the zones with different frequency reuse factors and transmit power levels. When the MS is in the edge zone, the MS is more likely to experience ICI than when the MS is in the center zone.
The sub-channels allocated to the MSs in the edge zone are called the edge sub-band and the sub-channels allocated to the MSs in the center zone are called the center sub-band.
FFR can be categorized as hard FFR or soft FFR. Hard FFR partitions the frequency band into the edge sub-band and the center sub-band. The edge sub-bands have a high frequency reuse factor, e.g., three, and the center sub-band has a frequency reuse factor of one. In other words, the sub-bands for adjacent edge zones are disjoint, while the sub-bands for the center zones of adjacent cells can be the same. To reduce the interference among the MSs in the center zones, the transmit power is usually less than the transmit power used for the edge sub-band.
In soft FFR, the MSs in the center zone reuses the frequency resource used by adjacent edge zones. To reduce possible interference to adjacent edge sub-bands, soft FFR uses a transmit power that is less than the power for the center sub-band that is not used for MSs in any cell edge zones. Soft FFR can be further partitioned into soft FFR A and soft FFR B, depending if there is a dedicated center sub-band or not. In the prior art, allocated bandwidth under FFR is fixed for a particular network.
There is a need for a dynamic FFR that can adapt to a changing network environment. For example, new cells may be installed or existing cells can be disabled temporarily or permanently. The traffic loads in cells can vary over time. The traffic load depends on the number of MSs and the amount of data that are communicated in a cell. Relay stations (RSs) may also be present in cells. Fixed FFR schemes with pre-determined resource allocation cannot meet such requirements.
Conventional FFR schemes require that any adjustment to bandwidth resources is the same for edge or center zones. All edge zones reusing the same frequency resource have to be extended or reduced by unit(s) of frequency resource at the same time. Similarly, all cells have to extend or reduce, if applicable, the center sub-band at the same time. Therefore, the cells using the same edge sub-bands have to have same allocation adjustment. Such mechanism is not able to satisfy the needs in case where different cells have different needs.